In an analog telephone system, the frequency band of speech transmitted through a telephone circuit is band limited to a narrow band such as 300-3400 Hz. Accordingly, the quality of sound of the conventional telephone circuit is not so good. In addition, in digital speech communication such as a mobile phone, since the bandwidth is limited as in the analog circuit because of the bit rate limiting, the quality of sound is not so good as well.
With respect to the problem, an audio signal restoration technique is known which generates or restores a broadband audio signal simulatively from a narrowband audio signal on a receiving side. The audio signal restoration technique, however, has been developed as a technique to be applied to audio devices or fixed telephones from the first, and supposes an environment that has no or little noise mixed into the narrowband audio signal to be restored.
On the other hand, as a simulative restoration technique of a broadband audio signal from a narrowband audio signal to be restored which has noise mixed, an audio band expansion device of Patent Document 1 is proposed, for example.
In the Patent Document 1, as preprocessing of the audio signal restoration, a component separation unit is introduced which separates the narrowband audio signal into a noise eliminated signal (audio component) and an extracted noise signal (noise component). The component separation unit comprises a noise eliminating unit, a differential processing unit, and a periodic component eliminating unit, and separates the narrowband audio signal into the audio component extracted by the noise eliminating unit and the noise component extracted by the differential processing unit and periodic component eliminating unit. Then, a post-stage of the component separation unit executes audio signal restoration for the individual components so that even if noise is mixed into the input narrowband audio signal, it can reduce unpleasant noise and improve intelligibility of the output broadband audio restoration signal.